Olefin polymers comprising ethylene and at least one or more .alpha.-olefin and optionally one or more diolefin make up a large segment of polyolefin polymers and will be addressed as "ethylene copolymers" herein. Such polymers range from crystalline polyethylene copolymers to largely amorphous elastomers, with a new area of semi-crystalline "plastomers" in between. In particular, ethylene copolymer plastomers are now a well established class of industrial polymers having a variety of uses associated with their unique properties, such as elastomeric properties and their thermo-oxidative stability. Uses of the plastomers include general thermoplastic olefins, films, wire and cable coatings, polymer modification (by inclusion in blends with other polyolefins), injection molding, foams, footwear, sheeting, functionalized polymers (such as by free-radical graft addition of polar monomers) and components in adhesive and sealant compounds.
Commercially prepared ethylene copolymers have been traditionally been made via Ziegler-Natta polymerization with catalyst systems largely based on vanadium or titanium. Newer metallocene catalyst compounds have received attention due to their ease of larger monomer incorporation and potential increases in polymerization activities. U.S. Pat. No. 5,324,800 describes metallocenes having substituted and unsubstituted cyclopentadienyl ligands which are suitable for producing high molecular weight olefin polymers, including linear, low density copolymers of ethylene with minor amounts of .alpha.-olefin.
Noncoordinating anions useful as catalyst components with such metallocenes is known. The term "noncoordinating anion" is now accepted terminology in the field of olefin polymerization, both by coordination or insertion polymerization and carbocationic polymerization. The noncoordinating anions function as electronic stabilizing cocatalysts, or counterions, for cationic metallocenes which are active for olefin polymerization. The term "noncoordinating anion" as used here and in the references applies both to noncoordinating anions and weakly coordinating anions that are not so strongly coordinated to the cationic complex as so to be labile to replacement by olefinically or acetylenically unsaturated monomers at the insertion site. U.S. Pat. No. 5,198,401 describes a preferred noncoordinating anion tetra(perflourophenyl) boron, [B(pfp).sub.4 ]-or [B(C.sub.6 F.sub.5) .sub.4 ]-, wherein the perfluorinated phenyl ligands on the boron makes the counterion labile and stable to potential adverse reactions with the metal cation complexes.
The utility of metallocene-based ionic catalysts in high temperature olefin polymerization is described in U.S. Pat. Nos. 5,408,017 and 5,767,208, EP 0 612 768, and WO 96/33227. Each addresses suitable metallocene catalysts for high temperature processes for olefin copolymerization. High molecular weight ethylene/.alpha.-olefin copolymers is an objective of EP 0 612 768 and is addressed with catalyst systems based on bis(cyclopentadienyl/indenyl/fluorenyl) hafnocenes which are combined with an alkyl aluminum compound and an ionizing ionic compound providing a non-coordinating anion.
As described above, a recognized problem for high temperature polymerization, particularly where significant content of comonomer incorporation in ethylene copolymers is to be sought, is an observed decrease in molecular weight, or increase in melt index (MI). Means of maintaining high molecular weights, or low M.I., in ethylene copolymers of low density (high comonomer content) while operating at economically preferable high polymerization reaction temperatures and high polymer production rates is highly desirable.